(1) Field of the Invention
The invention relates to a thin film transistor-liquid crystal display (TFT-LCD), and more particularly to a protecting structure for electrostatic discharge (ESD) in fabricating a flat panel of the TFT-LCD.
(2) Description of the Prior Art
During the process of fabricating a TFT-LCD, to prevent or to mitigate an ESD event is one of the most considerable problems. Otherwise, there usually accompanies a substantial drop in product yield. The ESD problems rise from a glass substrate of the TFT-LCD which is an insulating material (whose electric conductivity is about 10−14 S/cm) and has a pretty low electromagnetic decay rate. While in running a specific manufacturing process (e.g. dry etching) or while in conveying the substrate, many electrostatic charges can be accumulated around the surface of the substrate. Unless there exists a suitable discharging path on the substrate, or electrostatic charges may stay on the surface of the substrate. It is obvious that a bigger size of the TFT-LCD panel may accumulate a larger amount of the electrostatic charges. Therefore, to depart electrostatic charges from the substrate has become a major concern of the process of fabricating the TFT-LCD.
Referring to FIG. 1, a conventional ESD protecting structure along with a pixel array on the substrate is schematically shown. Also referring to FIG. 2, a schematic view of composition devices in a single pixel unit is shown. The pixel unit 10 includes a thin film transistor (TFT) 101, a capacitor 102 and a liquid crystal layer 103. In the pixel array, each gate (G) of the TFT 101 of the pixel unit 10 connects to a scan line 12. On the other hand, each drain (D) connects to a data line 16.
It should be noted that there exists two ESD protecting structures in FIG. 1. One is a first rake metal 14 formed aside the scan lines 12 (left-hand side in FIG. 1), and the other is a second rake metal 18 formed aside the data lines 16 (bottom side in FIG. 1). Both of the protecting structures are designed according to the same principal. For the second rake metal 18, an end point of each data line 16 is sharp in shape and each of which is facing, closely but not connecting, respective sharp tip of each short part 18a of the second rake metal 18. Upon such an arrangement, the sharp tip of the metal 18 can easily discharge the electrostatic charges on the glass substrate and thus problems that may be caused by the ESD can be better avoided.
Apparently, the foregoing protecting structure is used as an inactive way to achieve the ESD protection. Generally, the discharging effect between the data lines 16 and the corresponding rake metal 18 are hard to take place. Typically, a 7 kV is usually a minimum to initiate a discharging. Refer to FIG. 3 where a cross-sectional view of the area 19 in FIG. 1 is shown. As shown, the end point of data line 16 and the tip of short part 18a of the second rake metal 18 are all sharp in shape. These sharp structures can help to accumulate locally mass of electrostatic charges and then at some point to break down through the insulating layer 24 for performing the electrostatic discharge. It should be noted that ordinary materials for the insulating layer 24 can be SiO2, SiNx or SiON. Among those materials, bad conductivity are their common property. Therefore, discharging phenomenon in between is hard to occur, and thus a bad performance in preventing the ESD problem can be inevitable.
Empirically, the ESD damage event mostly occurred around the edge of the pixel array. To take the foregoing conventional protecting structure for example, the edge area of the pixel array is still in the display area 20. As the ESD damage event occurred, several pixel units 10 around the edge or display area 20 can be damaged to directly affect image display of the panel. Hence, the present invention provides a novel protecting structure to resolve the aforesaid ESD problem.